Feb 28 2019
According to researchers in Japan, Austria, and Germany who performed a study recently, the scattering of dark matter against one another may occur only when they hit the right energy.
Astronomers observed that the dark matter does not seem to clump very much in small galaxies, but their density peaks sharply in bigger systems such as clusters of galaxies. It has been a puzzle why different systems behave differently. (Image credit: Kavli IPMU—Kavli IPMU modified this figure based on the image credited by NASA, STScI)
The researchers’ concept sheds light on why galaxies, ranging from the biggest to the smallest, have the shapes they do.
An unknown and mysterious form of matter, dark matter contains over 80% of matter in the Universe currently. Although its nature is yet to be understood, dark matter is said to be responsible for creating galaxies and stars through its gravitational pull, which resulted in the existence of humans.
“Dark matter is actually our mom who gave birth to all of us. But we haven’t met her; somehow, we got separated at birth. Who is she? That is the question we want to know,” stated Hitoshi Murayama, study author and a University of California Berkeley Professor and Kavli Institute for the Physics and Mathematics of the Universe Principal Investigator.
Dark matter does not appear to fuse together as much as computer simulations proposed—a fact that is already known by astronomers. Assuming that gravity is the only force driving dark matter, never pushing and simply pulling, then the dark matter should be able to become an extremely dense towards the middle of galaxies. Conversely, in tiny faint galaxies known as dwarf spheroidals, in particular, dark matter does not appear to become as dense as anticipated toward their centers. It is possible to solve this mystery if dark matter scatters with one another similar to how billiard balls are scattered, enabling them to spread out in a more uniform manner following a collision.
However, one issue with this concept is that dark matter does appear to clump in larger systems like clusters of galaxies. It is not known what causes dark matter to behave in a different way between clusters of galaxies and dwarf spheroidals. In this context, an explanation developed by an international research team could possibly solve this mystery, demonstrating what dark matter is.
If dark matter scatters with each other only at a low but very special speed, it can happen often in dwarf spheroidals where it is moving slowly, but it is rare in clusters of galaxies where it is moving fast. It needs to hit a resonance.
Xiaoyong Chu, Physicist and Postdoctoral Researcher, Austrian Academy of Sciences
The phenomenon—resonance—occurs every day. For example, if individuals swirl a wine in a glass to absorb more amounts of oxygen so that it softens its taste and releases more aroma, then they have to identify the correct speed to circle the wine glass. Alternatively, they can dial ancient analog radios to the right frequency to tune into their preferred station. All these are instances of resonance, Murayama said. According to the team, this is exactly what dark matter is doing.
As far as we know, this is the simplest explanation to the puzzle. We are excited because we may know what dark matter is sometime soon.
Hitoshi Murayama, Study Author and Professor, University of California Berkeley
Yet, the researchers were skeptical that the data could be explained correctly with such a basic idea.
“First, we were a bit skeptical that this idea will explain the observational data; but once we tried it, it worked like a charm!” stated Colombian researcher Camilo Garcia Cely, a postdoctoral researcher at the Deutsches Elektronen-Synchrotron (DESY) in Germany.
According to the team, dark matter is capable of hitting the exact right note, and this definitely does not happen in an accidental way.
There are many other systems in nature that show similar accidents: in stars alpha particles hit a resonance of beryllium, which in turn hits a resonance of carbon, producing the building blocks that gave rise to life on Earth. A similar process happens for a subatomic particle called phi.
Camilo Garcia Cely, Postdoctoral Researcher, Deutsches Elektronen-Synchrotron (DESY)
“It may also be a sign that our world has more dimensions than we see. If a particle moves in extra dimensions, it has energy. For us who don’t see the extra dimension, we think the energy is actually a mass, thanks to Einstein’s E=mc2. Perhaps some particle moves twice as fast in extra dimension, making its mass precisely twice as much as the mass of dark matter,” stated Chu.
The next step of the researchers will be to locate observational data that support their idea.
“If this is true, future and more detailed observation of different galaxies will reveal that scattering of dark matter indeed depends on its speed,” stated Murayama, who is also heading an independent international team that is planning to do exactly this by utilizing the Prime Focus Spectrograph, which is being developed.
The instrument, worth US$80 million, will be placed on the Subaru telescope on top of Mauna Kea on Big Island, Hawaii, and will be able to determine the speeds of countless numbers of stars in dwarf spheroidals.